The present invention is related to inductor current controllers, and more specifically to solenoid current controllers.
It is generally desirable to implement actuation of a solenoid by providing an initial or pull-in period of time during which a maximum first current is effectively passed through the solenoid inductor to achieve initial actuation of the solenoid. Subsequently, a smaller magnitude solenoid current is implemented so as to maintain actuation of the solenoid wherein this occurs during a subsequent holding or hold period of time. In this manner, the efficiency of the solenoid controller is increased since only the minimum necessary holding current is utilized by the solenoid for maintaining solenoid actuation whereas a high pull-in or actuation current is initially permitted to insure the rapid response of the solenoid to a solenoid actuation control pulse.
Many prior circuits have implemented the general features of the solenoid current control system discussed above. Some of these systems determine the pull-in time, during which a high value of solenoid current can be drawn, by use of a monostable multivibrator. These systems typically utilize additional monostable multivibrators for implementing cycling of the solenoid current about a first high effective current pull-in level and then about a low holding current level. This cycling is implemented by essentially opening and closing the connection between the solenoid coil and a power supply. During the pull-in period solenoid current varies between maximum and minimum initial (pull-in) high current levels, and during the holding current period solenoid current varies between maximum and minimum lower holding current levels. The above referred-to copending U.S. application describes a similar system except direct control of the maximum/minimum current levels of the solenoid current is implemented by the use of separate maximum and minimum comparators which receive separate pull-in and hold thresholds during the pull-in and hold times, respectively. This results in more accurately controlling the magnitude of solenoid current.
In some prior systems, in addition to implementing a high maximum solenoid current during the pull-in period, to insure rapid turn-on of the solenoid in response to a control pulse, a high boost voltage has been applied across the solenoid during the pull-in period. This speeds up the initial actuation of the solenoid since this boost voltage is typically substantially higher than the normal voltage applied across the solenoid during the hold period of time. Typically, an auxiliary high voltage supply is initially applied across the solenoid during the pull-in period, and this is commonly termed the "boost" power supply. Previous implementation of applying a high voltage boost supply to the solenoid involved applying the boost voltage for the entire pull-in period. This is undesirable as it causes high voltage stress on the current switching elements that control solenoid current and the components in the boost voltage generator. In addition, this previous circuit design also reduces the time available for the boost voltage generator to recover between sequential pull-in times so that a high boost voltage is available for each sequential pull-in period. This is very important when the solenoid comprises a fuel injection solenoid and the engine into which the fuel is injected is operating at high speeds, thus requiring a rapid sequence of pull-in times.